Geology Reference
In-Depth Information
EXERCISE
8
Landslides and Avalanches
INTRODUCTION
Downslope movement, or mass movement, of earth
materials is taking place continuously at rates that vary
from the very slow creep of soil and rock to extremely
rapid rockfalls and avalanches. The movement may
involve only a few cubic yards or as much as 5 cubic
miles of widely differing consolidated and unconsoli-
dated materials. The primary purpose of this exercise
is to understand the nature of mass wasting and some
of the ways in which it impacts the human colony. We
will investigate the types and causes (including human
activity) of mass wasting and options for damage
reduction with examples of landslides from California,
Montana, Ohio, and Washington and snow avalanches
from Colorado and Utah. Snow avalanches are a major
winter hazard in mountainous terrain throughout the
world. More people than just skiers, snowmobilers,
and owners of mountain cabins or homes are at risk.
Because many avalanche paths cross highways or
railroads, snowslides may impact travelers as well.
Avalanche risks can be high throughout the year in
mountains with glaciers or permanent snowfields.
Geologists and engineers have classified slope
movements (e.g., Sharpe, 1938; Varnes, 1978).
Although numerous classification schemes have been
devised, the one that is probably used most today is
the scheme developed by the National Research
Council (Table 8.1).
The most common types of slope movement
are falls; slides, which are classified as rotational
(slumps) and translational; and flows (Table 8.1 and
Figures 8.1 and 8.2). The term landslide, which will
be used in this exercise, remains a widely used non-
technical term for most perceptible forms of down-
slope movement even though some by definition
involve little or no sliding. Landslides are a geologic
hazard that annually causes millions of dollars of
damage to structures and substantial loss of life. The
most destructive landslides in recorded history
occurred in Kansu Province, China. There more than
TABLE 8.1 Classification of Slope Movements
Type of Material
Type of Movement
Engineering Soils
Bedrock
Bedrock
Predominantly Coarse
Predominantly Fine
Falls
Rock fall
Debris fall
Earth fall
Topples
Rock topple
Debris topple
Earth topple
Rotational
Few units
Rock slump
Debris slump
Earth slump
Slides
Slides
Translational
Many units
Rock slide
Debris slide
Earth slide
Lateral Spreads
Rock spread
Debris spread
Earth spread
Flows
Rock flow
Debris flow
Earth flow
Complex
Combination of two or more principal types of movement
(modified from Varnes, 1978)
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